Finely divided silica product and its method of preparation



, Sept. 28, 1965 c. L. BAKER ETAL FINELY DIVIDED SILICA PRODUCT AND I I METHOD OF PREPARATION 2 Sheets-Sheet -1 Fil ed be}. 20, 1958 INVENTORS,

' Sept. 28, 1965* I c. L. BAKER ETAL 3,208,823 I FINELIY DIVIDED SILICA PRODUCT AND ITS I METHOD'OF PREPARATION Filed (m.- 20, 1958 2 Sheets-:Sheet 2 MOLECULAR 'AGGREGATES OF SODIUM SILICATE \OUTER ENVELOPE OF POSITIVELY CHARGED HYDRATED SODIUM IONS BUMPER RING OF WATER MOLECULES AGGREGATED COLLOIOAL.

IONS 0F SILICA (HAVING A RESIDUAL NEGATIVE CHARGE.)

-FIG.9

United rates 3,208,823 Patented Sept. 28, 1965 INTRODUCTION .This application is a continuation-in-partof our copending application S.N. 292,936 filed June 11, 1952 and our application S.N. 763,444, filed July 24, 1947, both now abandoned. In these prior applications we describe methods of obtaining finely divided solids by treating various inorganic colloidal solutions, such as solutions of sodium silicate, with coacervating agents and insolubilizing agents. In the present application we define more specifically and ingreater detail the critical conditions required to produce a finely divided silica having the properties set forth below. It is believed and intended that all of the disclosures of the above applications are either specifically included or incorporated by reference in the present application.

This invention generally relates to a novel method .for the manufacture of hydrated precipitated finely divided silica particles which are substantially free from silica gel. More particularly, the present invention relates to a method which involves the mixing together of an aqueous sodium silicate solution, a clustering agent, and an insolubilizing agent under certain prescribed conditions so as to produce extremely finely divided silica.

In addition, this invention encompasses novel silica products having unusual properties, characteristics and uses.

BACKGROUND AS TO PRIOR ART SILICAS As is well known, there are numerous industrial applications for silica, such as fillers and pigments in paints, rubber and greases. For many of these industrial applications it is highly desirable that the silica be in a very finely divided condition (e.g., below 1 micron in size and preferably below 0.1 micron),

Grinding is probably the most widely used and most satisfactory method for obtaining finely divided silica, but the grinding of silica to any high degree of fineness requiresthe expenditure of such a large amount of power vthat it renders the method impractical and particularly so when a fineness of less than about 1 micron is desired.

In attempting to grind silica to a size below about 0.1 micron it has usually been found that the physical character of the grinding surface itself acts to effectively prevent the attaining of such a fine subdivision.

Various other methods have'been suggestedfor the production of fine silica, such as chemical reactions in the vapor phase, chemical precipitation methods, subliming' methods, condensations from the vapor phase, methods involving gelation, etc. Most of these methods are expensive, however, and others have drawbacks which render them commercially unattactive. Nevertheless, in spite of these unfavorable features, several of these auxiliary methods have been used commercially owing to the great demand for pigments, fillers, reinforcing,

insulating, delustering agents and the like.

According to one of these chemical precipitating methods it has been proposed to produce finely divided silica by slowly adding a mineral acid or other precipitating agent to a sodium silicate solution containing a chemical agent such as sodium chloride (e.g. Kanhofer 2,438,560).

In this method, it has been considered necessary to add the precipitating agent slowly to the sodium silicate sot 2 lution in order to avoid the formation of a gelatinous precipitate, the thought apparently'being that if the precipitate should be formed gradually it would be nongelatinous. The precipitating agent was therefore added in a fine stream with continuous stirring during a period ranging from 5 to 10 minutes or more. It is evident that the particles produced at the start of the slow addition of precipitating agent are formed under instantaneous conditions of pH, concentration of silicate and ratios of all the reacting chemicals which are far different from the environmental conditions obtaining at the conclusion of the addition. The ratioof precipitating agent to silicate, for example, varies from zero to a maximum value during the course of the mixing.

Also, some prior art workers have added various chemical compounds to silicate solutions in amounts sufiicient to form a creamy suspension which is neutralized slowly to form relatively large particles (e.g. Moyer, 2,386,337 and Donau B.P. 653,993).

There are prior art references which advocate very rapid agitation during the addition of the various reagents, but in. these cases at least the final reagent has has been added to the vigorously agitated solution over a period of considerable time, such as several minutes or even several hours. It is evident that even with very rapid mixing and intermingling of the reagents under these conditions all of the precipitation cannot occur under exactly the same environmental conditions, i.e. the precipitate cannot be formed from a homogeneous solution.

OBJECTS The primary object of this invention is to produce a silica pigment of very small size. Another object of this invention is to produce a finely divided silica product having unusual utility as a rubber filler-and as a thickener for oils.

Other objects of this invention will be evident after reading the following description and claims, taken in conjunction with the drawings.

THE INVENTION BROADLY This invention broadly relates to a novel method for the manufacture of very finely divided silica particles which are substantially free from silica gel and which have a number of unusual properties. The instant novel method broadly involves mixing together and retracting the following three substances:

(1) An. aqueous solutionof sodium silicate,

(2) A clustering (or coacervating) agent which is capable of imparting a faint opalescence to the sodium silicate solution, and- (3) An insolubilizing agent which is capable of precipitating substantially pure silicafrom the mixture,

in such a manner that the pure silica separates under homogeneous conditions.

PROPOSED THEORY BEHIND THE INVENTION The preparation of finely divided silica by the instant process cannot be explained in simple terms according to present theories of physical and colloidal chemistry. Among other things, an exact scientific dissertation upon the present process would necessitate a lengthly discussion of such complex subject matter as electron theory,

protons, electron screening, sharing of anions, association silica and micelles of aggregated silica and gelation occurred.

of a'uniformly. small size.

(FIG. 8) with adsorbed ions of hydrated sodium." Because the-sodium silicate is completely ionized, other ions of sodium sufficient to neutralize the charge are located in a layer close to the surface of the silica micelles (FIG. 9) and prevent further aggregation. When, however, acidic ions neutralize the alkali ions, the silica and the silicate ions are free to approach closer until the whole massformsv a gel.

If, before the neutralization agent, a clustering agent is added, it-becomes possible to precipitate 'thelimited amount of silica in the clusters without forming'an ex: tensive gel and with-resulting preparationof a finely divided silica.- These clustering agents are known ascoacervating agents. It was'found, however, thatclustering agents into a sodium silicate solution was not the answer to providing finely divided silica products, even though it did avoid gel -formation. Specifically, it was found that the introduction of the clustering agents caused the molecular aggregates of sodium silicate to cluster together soq'uicklythat in an extremely short period of time the clusters of aggregates would grow .so rapidly and so large that upon contacting with an acid or other insolubilizing agent the resulting silica product would nearly valways be greaterthan 1000 millimicrons in site. On

. the other hand,'if too small an'amount of clustering agent Was added, very little or no clustering tookplace .The presentinventors ultimately discovered that one 'of the unexpected secrets of producing 7-100 millimicrons materialwas that the amount of the clustering agent had to. be critically controlled. It was unexpectedly discovered that the correct proportion of clustering agent'to silicate solution could be estimated very closely by a simple test in which the clustering agent is slowly added in measured quantities to a pre-determined quantity of silicate solution the mere introduction of 4. not having an average particle size below 100 millimierons. However, when it is desired to produce a final .silica product having an averageparticle size below about 20 millimicrons, it is desirable to employ aqueous sodium silicate solutions having a weight percent ratio of'Na O to SiO of from about 1:1 to 1:4, generally from about 152.4 to 1:3.4. As a general rule, the higher: concentratrons of SiO, are employed with the lower ratios of Na o t0 siog.

. Although'a number of commercial sodium silicates are I available which have the above described'properties, those with sufiieient time being given between each addition for the mixture to come substantially to equilibrium, the clustering agent being added untilv the first appearance of a faint opalescence in' the mixture.

The change from non-opalescent to an opalescent sodium silicate solution appears to be one of those physical phenomena which occurs at a single point of concentration rather than over a range of concentrations. By stopping. the addition of the ,clusternig agent just at this critical point of concentration we were able to obtain clustersof the desired size, In order to convert these clusters to silica, an insolubilizing agentmustbe added. If the insolubilizing agent is added slowly, as the prior art teaches, the gradualchange in the concentration of, the reactants and the reaction products will cause a large proportion of the clusters to either gel or grow rapidly in' size, thus preventing the precipitation of silica particles It is likewise-true that so long as the insolubilizing agent is added to the silicate solution slowly, the above mentioned difficulties of, gel 1 formation and the formation of large particle size .Will

occur even though the slowly added insolubilizing agent isthereafter rapidly intermingled with the silicate solution after it has been added. Unexpectedly, it wasfound that n conditions and further growth of clusters did not 'occur and silica particles of a uniformly small size and other characteristics resulted. 4

THE SODIUM SILICATE SOLUTION The aqueous sodium silicate solution may have a weight percent ratio of Na- O to SiO- within the range of from about 2:1 to 1:4 and the concentration of sodium silicate maybe varied from about,0.5 to 30%'Si0 when the process is being carried out merely to obtain a silica'prodsodium silicates manufactured bythe Philadelphia Quartz Company under the following trademarks have been found to be most satisfactory:

I THE CLUSTERING AGENT Although the term coacervatingagent 'is probably the most scientifically definitive term insofar as this, invention is concerned,.it has been found preferable to use the term clustering agent in its place since this latter term seems to more vividly describe the physico-chemical, reaction which actually occurs. Accordingly, when-t ever these terms are used in the subsequent description and/orclaims it will be understood that they are meant to be synonymous with each other. Among the clusteringagents which have been successfully "used in ourpresent'novel process there may be mentioned: I

(a) Completely water-miscible organic-liquids, such as methyl alcohol, ethyl alcohol, acetone, ethylene glycol, monoethanoi amine, ethyl ether of ethylene glycol,

etc.; a

(b) Ammonium hydroxide and its water soluble deriva-' tives such asNH Cl, (NH SO (NH );CO

(0) Highly soluble uni-univalent salts such as NaCl, KBr,

NaNOitKCl; and

(d) Highly soluble uni-divalent salts such assodium sulfate and sodium carbonate.

Mixtures of coacervating agents are more difficult to control and to treat for the recovery of valuable materials, but in some cases there is an advantage such as, for example, the reuse of a filtrate containing both ammonia vand sodium chloride or sodium carbonate.

The clustering agent must be a hydrophiilic material which tends to reduce the effective charge on the colloidal particles in the sodium silicate solution and/or which reduces the effective dielectric constant of the medium betweenthepa'rticles. The ciustering agentneed not be an electrolyte since water-soluble or wateramiscible hydr'ophillic organic materials in general are capable of functioning as clustering agentsif added in sufficient quantity. The clustering agent should not form a precipitate with water and preferably not with a solution of the insolubilizing chemical. If the insolubilizing agent reacts with and forms a precipitate with the clustering agent this precipitate must bemore soluble than SiO, so that the latter is formed preferentially when the three solutions are mixed.

THE INSOLUBILIZING AGENT The insolubilizing agent used in our new process should preferably form no precipitate with either water or the clustering agent and must be sufliciently acid when retions. v

REACTANT AMOUNTS, REACTANT RATIOS AND acted with a sodium silicate to produce an insoluble precipitate of silica, silicic acid or silica. gel. It may consist of any of the mineral or organic acids, acid anhydrides or acid salts or mixtures thereof whose cations form soluble silicates; in fact any acid or acid material which is stronger than silicic acid can be used. Examples are H3PO4, H2804, HNO3, C02, S02, C12, P205, ammonium carbonate, ammonium chloride, ammonium nitrate, salts of organic cations such as basic dyes' and quaternary ammonium compounds, NaHCO NaHSO KHSO and organic acids such as acetic, oxalic, tartaric, citric, formic, fumaric, etc.

An advantage in the use of a carbonate ion as the insolubilizing agent is that less CO equivalent is required than in the case of sulfuric acid and only a little more than the equivalent of HCl. In addition CO is readily available from furnace stacks and can be absorbed by ammonia solutions in sufficient quantity for use. The ammonia and CO can be recovered in stripping towers and reused sothat a continuous process is possible in which all of the reagents are reused except the silica product. Na CO and NaHCO which are formed can be returned to the solution of clustering agent or they can be fused directly with quartz to make fresh soluble silicate solu- REACTION RATES It is evident that in our process two difierent reactions occur. The first is incipient clustering (or. the development of opalescence) and the second is precipitation or insolubilization. These two reactions can be conducted sepsarately or simultaneously. The two reactions occur at dif- 'ferent rates since they are controlled by ditferent phenomena. Our' tests indicate that the precipitating or insolubilizing reaction usually occurs somewhat more slowly at ordinary temperatures than the clustering reaction,

which may be due to the fact that the former requires a reaction'at the particle surface changing the electrical attraction. Use is made of this fact when the precipitating chemical and the clustering agent are added substantially simultaneously to the silicate solution. If the insolubilizing reaction were the quicker a gel would be formed by I this procedure.

. The relative rates of reaction of the two steps vary with temperature. [It is likely that above 50 C. the insolubilization reaction is the more rapid. Thus the temperature is a very important consideration when selecting the proper proportions of reagents. The dilference in temperature eitect is apparent from the fact that the solution temperature decreases on the addition of NH OH and then returns to a temperature higher than the initial one on the addition of H 50 for example. Common salt (NaCl), likewise, causes cooling. A solution of NH Cl, on the other hand, will cause cooling when added to a water solution at 25", but will cause heating when added to a solution at 30 C. The alcohols also tend to warm the solution on mixing. Insolubilizing agents, of course, tend to increase the temperature of the solution through the heat of neutralization. In addition, H 80 particularly causes the solution to warm by the heat of dilution. These temperature effects may explain the differences in results which are sometimes obtained upon change of insolubilizing or clustering agents.

The quantity of insolubilizing agent employed should usually be suflicient to substantially completely precipitate the silica present in the solution. Otherwise the process is uneconomical. A simple way of testing whether sufficient insolubilizing agent has been added isfto filter ofi the precipitate and then to add more insolubilizing agentto the filtrate. If a secondary precipitate is formed in the 'filtrate this demonstrates that more should have been added previously. When non-alkaline clustering agents are employed it is sometimes sufiicient to add an amount of insolubilizing agent which is chemically equivalent to the Na O of the silicate. As a general rule, however, it requires from about to 250% of this chemical equivalent to completely precipitate the silica. An excess over the exact requirement does no harm; it is merely uneconomical. A multiple precipitation may sometimes be useful. If insufiicient insolubilizing agent is added to a properly balanced sodium silicate solution containing a clustering agent, the silica particles will usually be in the larger size range. If, after this precipitate of larger particle sizes is-filtered oif, additional insolubilizing agent is added, the filtered precipitate which results will usually be of smaller, more uniform size. In both cases, the insolubilizing agent should be mixed in within a period not substantially exceeding 5 seconds.

If the clustering agent is mixed with the silicate solution in an initial step in an amount sufiicient or nearly sufficient to form a faint opalescence, a mixture is usually obtained which is stable isothermally. Such an activated solution may be termed a protocoacervate."

In our process the correct proportions of clustering agent to sodium silicate solution can be estimated rather closely by first of all establishing a reference point by a simple test in which the clustering agent is slowly added in measured quantities to a predetermined quantity of the silicate solution with sufiicient time being given between each addition for the mixture to come substantially to equilibrium, the clustering agent being added until the first appearance of a faint opalescence in the mixture. The so determined ratio of clustering agent to silicate solution may be called the equilibrium opalescence ratio and, for purposes of comparison, the amount of cluster- I fore the appearance of clustered particles in the dark field I of an ultramicroscope. Thiscan be accomplished either by (1) using a quantity of clustering agent which is less than the 100% ratio or (2) by using a quantity of clustering agent equal to or greater than this 100% ratio but mixing in the insolubilizing agent before any opalescence has developed. When the latter expedient is employed best results are obtained with the use of a slow-acting clustering agent, such as brine or ammonia. With these slow-acting clustering agents it is possible to add a quantity substantially greater than the 100% ratio provided that the insolubilizing agent is added promptly after the addhition of the clustering agent or simultaneously therewit In one series of tests, using method (1) above, enough ammonia was added (as the clustering agent) to a sodium silicate solution (having a ratio of Na O to SiO of 1:2.4) to produce the first appearance of a faint opalescence. This mixture was allowed to stand for a period of 30 minutes before the addition of an insolubilizing agent. It was found that the quantity of ammonia could be reduced to 80% of the equilibrium opalescence ratio and an excellent product was obtained at this ratio. In other tests, using method (2) above and employing a mixture of ammonium hydroxide and carbon dioxide as a combination clustering-insolubilizing mixture, it was possible to increase the proportion of ammonia to 485% of the equilibrium opalescence ratio. In other words, under certain conditions, it is possible to employ from about 80 to 500 percent of the quantity of ammonia which is required, when slowly added to a silicate solution, to produce the first appearance of a faint opalescence in the mixture. The corresponding range for NaCl is from about 30 to percent. When three solutions are used in the mixing procedure and the clustering agent is all solubilizing chemical in a two-jet system,

added to the silicate solution before the insolubilizing agent is mixed in, the operative opalescence ratios range "from about 30% to 150%. Where the clustering and in 'solubilizingagents'are used in admixture the operative range; of op'alescence ratios is from about 20 to500 percent. The exact ratio which should be used in order to produce a product of optimum properties for any given purpose will depend upon the silicate ratio, the SiO: con- .centration of the silicate solution, the rapidity and type of mixing employed and uponwhat insolubilizing agent is; used. Some preliminary-testing is usually desirable in order to obtain best results.

We have found that in carrying out our process, the insolubilizing agent is always added to the solution of soluble silicate either with or after the clustering agent-and that the clustering agent is always present in an amount sufficiently low that there results a translucent solution which becomes opaque within a finite. time interval less than seconds if a product is to be obtained having'an average particle size below 100 mu. On the other hand, we have found that the'development, of gel particles can be avoided by maintaining sufiicient clustering agent in 'this translucent solution.

a :cRITIcA ITY 1N MIXING THE Ra ser-AN S In order to form finely divided silica under homogeneous'conditions, according to the teachings of thisinvention, the mixing and reacting of the threereactants should i take place under the following conditions: i

"(a) The 'en'vironmental conditions should bemaintained 8 of mixing a silicate solution and a clustering agent in a Y tube followed by mixing the resulting mixture with an insolubilizing chemical in an ejector, a continuous filter being employed for recovery of' the silica product,

FIG. 5 is a similar showing of. how a silicate solution can be mixed continuously in. a Y tube with a pre-mixed mixture' of clustering agent and insolubilizing chemical,

followed by continuous agitation and mixing caused by passing the mixture through a series of constrictions,

FIG. dis a similar showing of how the silicate solution can be jet-'mixed with a jet of a preformed mixture of clustering agent and insolubilizing agent, while FIG. 7 'showsthe use of a constriction and tubularconnections' to produce thorough mixing and agitation of the final mixture while in transit.

.. In the various figures of the drawing the reference numeral 1 denotes the silicate solution, 2 the clustering agent and 3 the insolubilizing agent. In FIG. 1 three jets are employed but the jet of silicate solution and the jet'of clustering agent meet and unite before the jetof insolubilizing chemical contacts. the. mixture. The final mixture still in the. form of a jet strikes the battle 4 before falling'into the container 5. In FIGJ2 the silicate solution and clustering agent are mixed in the Y tube 6, the resulting mixture in the form of a jet being mixed with a jet of insolubilizing .chemical before the mixed jet strikes the batfie; clustering agent are mixed in a vessel 7 provided with an agitator 8. The mixture leaves the mixing vessel in the form of a jet and this, jet unites with a jet of insolubilizing 1 chemical before the resulting mixture reaches theconsubstantially uniform in the mixture while the SiO: particles are in the process of formation by'mixing the clustering agent with the sodium silicate not later than.

. vthe insolubilizing agent is'mixed therewith; and

I 'i-(b) 'The. mixing-in of the insolubilizing agent should be completed within a 'period 'not substantially exceeding 5 seconds and before the appearance of any substantial amount of an irreversible precipitate in the solution.

'As noted earlier, the aforementioned reactants can be mixed in several different ways. Thus the three reactants can be mixed together simultaneously, or .the silicate solution can be mixed with the clustering agent first to form-a faintly opalescent solution, before the addition of the insolubilizing agent, or the clustering agent can be mixedwith' the insolubilizing agent before mixing v with the silicate solution; It is essential, however,-to prevent the. insolubilizingagent from becoming mixed with the vsilicate solution substantially before the latter has been mixed with the clustering agent. Also, as mentioned previously, it is essential in the step wherein the insolubilizing agent is reacted with the silicate solution that thorough-mixing be substantially completed before the forrnation of substantially any irreversible precipitate, i.e.

that the mixing be substantially instantaneous.

Several continuous mixing procedures are availablewherein the ratios of insolubilizing agent to silicate solution are inherently maintained constant during the mixin'g. Some .of these procedures are illustrated diagram .matically in the accompanying drawings. In this showing:

FIG. 1 is a diagramatic showing of how the three components of our mix can be contacted and mixed in the form of three jets, I

' FIG. 2 .is a similar showing of how a silicate solution and a clustering agent can be mixed by means of. a Y tube and the .resulting mixture contacted with an .insolubilizing agent in a two-jet system,

FIG. 3 is a similar showing of .and a clustering agent can be mixed and agitated in a vessel and the resulting mixture contacted with an in- FIG. 4'is a similar showing of an improved method how a silicate solution tainer 5.

i In FIG. 4 a continuous processis illustrated wherein asilicate solution and clustering agent are mixed in the Y tube 6 the'discharge from which passes tangentially into the upper chamber of ejector 8. The insolubilizing chem-.

ical-'isfed into the nozzle 9 of the ejector passing around the spiral baflle 10 and thereby acquiring'a circulatory motion before passing through the nozzle. Owing to the circulatory motions thus produced a very thorough and substantially instantaneous mixing is produced before the resulting mixture passes into the pan ll of the continuous filter shown generally at 12. The rotary filter.

drum 13 filters off residual liquor from the silica particles and the latter are scraped off by the knife 14. The particles can be washed while on the drum if desired.

In FIG. 5 the clustering agent and insolubilizing chemical are firstmixed in the Y'tube 6a and this mixture is mixed with the silicate solutionin asecond Y tube 15, after which the resulting mixture passes through a series of constrictions 16 to produce thorough mixing.

In FIG. 6 a jet of silicate solution ismixed with a jet of p a pre-formed mixture of clustering agent and insolubilizing agent formed in 7a, the resulting mixture falling into container 5.

In FIG. 7 the three solutions are mixed in Y tubes and the finalmixture is then passed first through a constriction 16, then through a flexible tubing 17 of large size and finally through a tubing 18 of medium size. In other cases, a length of narrowtubing is attached directly to mixing is, highly advantageous in the step wherein the insolubilizing chemical is mixed with the silicate solution and is also advantageous but of less importance in the step wherein the silicate solution is mixed with the clustering agent.

ln'FIG. 3 the silicate solution and- In order to obtain equilibrium in the short time available, particularly at high concentrations of S or high opalescence ratios, it is important to develop a very rapid 'r'nixing or in other words, a very high intensity of mixing Various expedients such as the use of jets or Y mixing devices develop very rapid or intense mixing it sufficient pressure is provided. The sufficiency of a given pressure will depend on the viscosities, velocities, and the angles of intersection and other properties of the stream which, for tube flow, may be measured by the Reynolds number. In devices employing tubes, turbulent flow with a Reynolds number of at least 10,000 is required. In

. other types of mixing, high speed pumps used below the limit of their capacity may provide sufficient intensity.

In our experiments we have developed Reynolds numbers of 100,000 or more and have used jets at pressures overv 40 p.s.i. and narrow tubes at presure's of 200 p.s.i. and

stirrers rotating at 7,000 rpm. or more. We see no ob-' 'stacle to the use of higher pressures or greater intensities except the physical properties of the equipment used. Ourtests indicate that the more intense and complete the i mixing, the finer the particles produced and the more a 5 seconds.

. Best results are obtained when use is made of a concurrent flow mixing technique, described-more specifically I hereinafter, wherein the proportions of insolubilizing agent to silicate are maintained substantially constant during the mixing. When this type of mixing is employed, substantially instantaneous mixing can be. accomplished inherently. The requirement that all of the silica particles be formed in the same environment obviously preeludes pouring the insolubilizing solution slowly into a beaker containing a mixture of sodium silicate solution and clustering agent or vice versa, as described in the prior-art. We have discovered that these conventional mixing procedures invariably produce products containing substantial proportions of silica gel. Moreover, the products produced by these prior art procedures contain silica particles of widely varying size whose average size is substantially greater than that of the products produced by our improved process. In our tests with our new mixing technique we have been able to produce some products having an average particle size of as low as 7 mu or less.

' By holding concentrations and operating conditions within preferred critical limits we have been able consistently to obtain products having average particlesizes below mu and-substantially free from silica gel.

The ratio of clustering agent to silicate solution employed in the process and the speed with which the clustering agent acts will determine how soon the insolubiliz ing chemical must be mixed in. If the opalescence ratio is low, for example, if it approaches percent, it is possible and sometimes advantageous to mix the silicate with the clustering agent and then to let this mixture stand for from a few minutes to several hours before mixing in the insolubilizing agent. This is particularly true when slowa clustering agent which is at least moderately slow in action, such as NH or NaCl. We have obtained our best results using CO or CO dissolved in ammonium hydroxide. It therefore appears that a slow-reacting insolubilizing agent gives better results than a quick reacting agent. This may be due to the fact-that it is possible to obtain complete mixing in the case of the slow-reacting agents before their action becomes effective. If the opalescence ratio is exceeded it is usually necessary to mix in the insolubilizing agent simultaneously with the clustering agent and before the appearance of opalescence.

As indicated above it may be a matter of choice whether the clustering agent is mixed With the silicate solution before or simultaneously with the insolubilizing agent, this depending upon the opalescence ratio and the conditions employed. When the clustering agent is mixed in a beaker with the silicate, for example, local high concentrations may produce clustering. But this temporary clustering disappears upon further mixing. The manner in which the clustering agent is mixed with the silicate solution appears to be of less importance than the manner in which the insolubilizing agent is incorporated in the mix. The principal requirement seems to be that the mixing of clustering agent and silicate solution should be completed before the appearance of any trace of permanent precipitate in the mixture. This is essential to produce fine silica particles of relatively uniform size. Rapid and complete mixing produces best results and it is advantageous but not necessarily essential to keep the ratio of clustering agent to silicate solution substantially constant as mixing proceeds. One of the simplest ways of holding this ratio constant is to pour the clustering agent and the silicate solution simultaneously into a beaker, the pouring rates being controlled so that the relative proportions remain substantially constant throughout.

the mixing. On a larger scale the two solutions can be piped into a mixing .tank with the flow in each pipe controlled to produce the correct ratio. t

In the case of the mixing step wherein the insolubilizing agent is brought into reaction with the silicate solution containing a clustering agent at the point of opalescence it is essential that mixing be completed substantially instantaneously before the appearance of any substantial amount of irreversible precipitate in order that the environmental conditions be maintained substantially uniform while the particles are being formed. In the case of slow-acting insolubilizing agents or low opalescence ratios this can be accomplished with fair success by pouring the insolubilizing agent quickly into an agitated beaker containing a mixture of clustering agent and silicate solution or by pouring all three into the beaker simultaneously. Pouring (i.e., mixing) should not take over about 5 seconds and, as indicated, it should be completed before the appearance of any substantial amount of, irreversible precipitation. Agitation of the batch should be continued until it thickens. When the insolubilizing agent is mixed with the silicate solution by pouring them simultaneously into a beaker it is evident that the ratio of these two reagents is maintained substantially constant during the mixing. Inherently the environmental conditions are maintained substantially uniform during the formation of the silica particles. It is also possible to accomplish this result by first mixing the clustering agent with the insolubilizing agent and pouring this mixture into a beaker simultaneously with the silicate solution. Better results are usually obtained under these conditions. On a larger scale the solutions or mixtures can be piped rapidly and simultaneously into an agitated tank. They also may be run in at opposite sides of the tank filled with a diluting agent which may contain clustering agent and brought into reaction in a central area where the conditions of precipitation are maintained constant. These mixing methods, are, of course, batch processes.

A further very surprising discovery in accordance with this invention is that the mixing conditions which prevail after the silica is precipitated a'revery important insofar as producing silicas which will result in a high modulus -s'ile strength as well as its other properties.

when compounded with rubber. We have found thatif vigorous mixing is continued after the point of precipitation of the silica (that is, when the solution thickens or sets up) the resultant-silica will create high tensile strength but not the highest modulus. Although the silicas produ'ced by our invention in general havea relatively high modulus, we have foundv that inixing should be'stopped completelyfor at least 30 seconds and preferably longer immediately after the solution sets up. in order to develop the very highest modulus properties. However, 'ajgentle agitation may. be permissible. I s

TREATING THE FRESHLY PREClPlTATED SlLlCA There is a distinct difference between the prodrictsiof tray drying and spray drying. In tray drying,'the product necessarily remains at about the boiling pointof water for periods ranging from below about one minute to several hours even though the temperature of the surrounding air may be well above .110 C. and the layer orlu'mp thickness-may be as low as a sixteenth of an inch.; Our product dried under such conditions tends to. have a lower surface area and increased aggregation.

pelled as small particles which ,may remain in an atmosphere no warmer than 105-.C. for substantially lessthan oneminute has higher surface areas and is less aggregated.

The washingwill also helpto controlthe effect of the drying temperatures. 'If'the' Na O is reducedbelow-about 1% there is'ilittle difficulty with increased aggregation and reduction in surface area by fast drying. When silica is washed with distilled water it is easy to. prepare aproduct having 99.5% Si on the anhydrous basis.

However, when washed with raw water the silica has a strong attraction for cations and the HF residue rises rapidly. This may rnean that the silicacoutent of the final anhydrous product'will be-as low as about.95%.

If an acid Walsh is included before drying so thatthe pH is reduced below neutral, the surface area will be very much greater. However, the product may be acidified and then broughtback to an alkaline condltion before drying if a low surface area is required.

USE'OF'OURNOVEL SILICA IN RUBBER This invention is directed'to a new hydrated precipitated silica formed by a new process. This product is different from any silica precipitate known heretofore as shown by its characteristics. It is particularly suited to reinforcing natural rubber. It is well-known that. the

impart really high modulus vaules to a compounded rubber while maintaining satisfactory tensile values.

On the other hand, spray dried product, that isproduct which ispro- -ber indust r'y, when suitably compounded in. our standard formulation (described under test methods) will produce vulcanized rubbers having at 'least the following P.s.i. 300%. modulus 1400' Tensile. strength 4000. Tear resistance 700 Prior-art sllicas have approached one or possibly two *of these values. but it is believed that this isthe first instance when all three of these valueshave been obtained ment of our silicas was about the same for GR-S as for natural rubber. Therefore, most of our testing wasdone GR-S type of synthetic rubber is rather easily reinforced ASTM linseed oil adsorption, grs./

For example, in comparing natural vs. synthetic rubber we note that natural rubber possesses a high tensile strength but requires reinforcement to improve its abrasion resistance as a function of increased modulus and tear resistance. Synthetic rubber on'the other hand require a high degree of reinforcement to improve its tenproperties of vulcanized, non-reinforced'natural and synthetic rubber are as follows:

' GR-S Typical Natural 300% modulus, p.s.i. 275 200 Tonsil strength, p 4, 000 1,300 'lear resistance, p. 275

Elongation, percu Shore harduess Rebound, percent the following properties:

with the natural rubber as a standard. It is recognized that material which reinforced natural rubber-will correspondingly improve GR-S rubber.

We define a high modulus silica asa product which, when incorporated in our standard formulation, permits a tensile strength of at least 3700 p.s.i. and a modulus above 1300 p.s.i. at 30% extension when vulcanized at our standard temperature using the optimum concentration of accelerator.

Our preferred high modulus'material has approximately Particle size 10-20 mu. pl-l 6-10. SiO content, percent (anhydrous basis) Greater than 95. Free water, percent 2-10. Bound water-mols of silica/inol of H 0 4-8; Filler form Porous'aggregates. Surface area, mfi/gr. -250. Na O content, percent 0.1-1.5.

Tensile strength in natural rubber,

p.s.i Greater than 3700. Tensile strength in GR-S rubber,

p.s.i Greater than 2500. 300% modulus in natural rubber,-

p.s.i Greater than 1300.

gr Greater than 1.5. 325 mesh wet sieve residue, percent 0.00.-

Grease thickening power, mm./l0 Less than 275. HP residue Less than 4%. Ignited loss, percent 5-15. Refractive index About 1.46.

It is, of course, recognized that high modulus may be obtained in a compounded rubber composition by varying the additives and the processing conditions, but at the same time the tensile strength will be greatly reduced. We have'for the first time developed a precipitated silica which willdevelopa high modulus above about 1300 p.s.i. at 300% elongation under conditions which develop the optimum tensile strength. For our high modulus silica, the optimum tensile. strength loading is between 10 and 15 volumes per volumes of rubber. The optimum loading is somewhat higher for a high tensile type silica having a particle size in the range of 10-30 mu.

Our silica also has higherlinseed oil adsorption values than other precipitated silicas, using the standard .ASTM

13 procedure employed for control purposes in the carbon black industry. We believe that this is indicative of high structure and correspondingly high' modulus.

Our procedures which involve the co-precipitation of latex and SiO; are of particular importance to the rubber industry since an extremely uniform mixture is produced without the use of any costly milling procedure. Furthermore, the mixture is produced at a time when the particles of both rubber and silica are in a peculiarly active state.

OTHER USES AND PROPERTIES OF OUR NOVEL SILICA,

The pure silica products obtained inour invention are uniform in particle size and shape, usually amorphous, the bulk being spherical andcharacterized by their unexpected softness.- After washing with distilled water and drying at 100 C. they have the following physical prop- I. The products produced by the present invention pass a 325 inesh Wet sieve to the extent of about 99.8 percent -Or better. Our best products lease less than 0.01 percent retained on the 325 mesh screen.

It is believed to be evident from the above description that'the novel products produced by our method have many important industrial uses, such as fillers, pigments, delustering agents, diluents, dust insecticides, components of printing inks, polishing composition, fertilizers, paints, etc., reinforcing agents, filter aids, insulating compounds I and the like. The silica products of our invention are believed to have all-around properties which are superior to those of any ot-the somewhat similar products which have previously appeared on the market.

.Our products are especially satisfactory for thickening oil for high temperature lubrication.

We have also found that our new process is especially valuable in giving control of. structural characteristics of the product, which makes it especially useful for providing a coating of very fine hydrated silica particles on fibers and-other inert-particles. Thus, a cotton'fiber may Thermal conductivity of the silica was determined by the method described in Industrial and Engineering Chemistry 31, 827 (1939). The water around the guard ring and calorimeter was kept at the same and a constant temperature through a controlling heater. a

The value for the hardness mentioned in the above description was determined by rubbing 0.1 gram of the SiO product between two zinc plates for one minute and determining the weight of the ,zinc removed from the plates by attrition per gram Of SiO employed.

The particle size was determined by the P. and G.

I method, described by Pechukas and Gage,-Ind. Eng. Chem.

be made resistant to slippage or dye pigments may-be extended.- v

TESTING AND ANALYTICAL PROCEDURES USED 1N EXAMPLES In order to more clearly understand the examples which are to follow, the various testing and analytical procedures employed in these examples are indicated below.

We have found a rather simple way of testing silica products for the presence therein of silica gel. .This is done by subjecting the products to a 325 mesh wet screen test, after grinding under standard conditions. Any residue left on the screen can be considered to be silica gel.

: This is the same test which has been approved as standard for carbon black and is known as ASTM Test D185-45 see American Society for Testing Materials Standards of 1949 (part 4), page 163.

Analytical Edition, 18,- 370 (1946), using a pressure of 35 psi. for plug formation and corrected by an empirical chart drawn from determinations by an electron microscope.

pH is determined by the Wiegand method (Ind. & Eng. Chem. vol. 29, p. 953, 1937) widely used in the carbon black industry. One part of the pigment is mixed thoroughly with 4-10 parts of water and the mixture is heated and boiled for at least one minute. It is then cooled to room temperature, settled, decanted and the sludge is tested with Hydrion. paper. This correlates with the electro-metric meter determination.

I Titratable Na O is determined by taking one gram of silica with 50 ml. of water. The mixture is heated to boiling, cooled to room'temperature and titrated with 0.2 N HCl to a Methyl Orange end point.

The surface area may be measured by the Brunauer- Emmett-Teller method described in J. Amer. Chem. Soc. 60, 309 1938). Actually, most of our determinations were made by a variation of this method described by W. B. Innes in Analytical Chemistry 23 (5 759 (1951).

-The HF residue is determined by standard chemical analysis, i.e., the silica is determined gravimetrically and then volatilized with HF and the weight of the ignited residue is deter-mined. I

The refractive index was determined using the Backe test with standardized immersion liquids as described by Chamot and Mason in Handbook for Chemical Microscopy, vol. 1,. John Wiley and Sons, 1930.

The bulk density or apparent density was determined by the method indicated by Scholes on p. 7 of the Handbook of Glass Industry," 1941, with the variation that the ml. graduate was filled with the fine silica powder and then the bottom of the graduate was tamped lightly on a large rubber stopper until no further decrease in volume was evident. The tamping was then continued for 100 more strokes. The weightof the sample divided by the final volume gives the bulk density.

The filler form is found either with a very high power microscope or from electron-microscope photographs.

Free water is determined by drying at- C. for 20 hours in a laboratory oven. The thickness of the layer should not be more than one-half inch and some provision should be made for moving the air in the oven. It should be dried to a constant weight.

Bound water is determined by substracting the free water from the total ignited loss. Ignited loss is found by heating the pigment to above 800 C. to constant weight. p

The grease penetration test is described in ASTM D-2l752T or D-2l7-48 (ASTM Standards, 1949, Part 5, p. 816).

The linseed oil adsorption is determined by the standard ASTM test D28l-3l (ASTM Standards, 1949, Part 4, p. 169) in which the pigment is rubbed out in linseed oil.

In the testing of our product with natural and synthetic rubbers, the standard ASTM Procedures which were in I effect at the time, as for instance, Dl5-55T and D15-41,

' ASTM Standards,

1949, Part 6, p. 775, were followed in general.

, Most of our test work was donewith natural. rubber, using the following standard formulation:

Components Recipe, Final composition 4 grams V Natural rublierf 101.4 100 parts. 60% zinc oxitlotnnstor hotcln. 122.1) '5 parts 2110.. Y

50% sulfur mastcrbotch.-.. 13. 2 2 parts sultun. Santocure muster butt-l 8. '53. 5 0364.5 parts Stmtocure. Stcurex bonds ti. 0 3 parts stearlcpcld.

Silica reinforcing lillcr 04 parts (20 volume v loading).

Usually the natural rubber was a niisture cl higli-grudepnlc,

creporubbcr and high-grade smoked sheet rubber. i The amount of batch rcpared depended on the. mill to be used, tltonumbcr ol' accelerator evl variations to be determined and the numberot cure times necessary. In general, it was approximately 400-gran'is or rubber. i

A master. batch is a premixed combination of natural rubber, e/g. 50%- sulfur'and 50% natural rubber. I Santocure is a rubber accelerator sold by the Monsanto Chemical Co. and is said to be a condensationfproduct of mercaptobenzothiazole with cyclo hexyl amine.

' --S tearex beads area hydrogenated fish oil sold by the Emmy and Smith Co. and correspond to steari'c 'a'c'id. A fine grade of zinc oxide 'sold'by as XXR4 is preferred.

g theNew Jersey Zinc Co.

A vfine grade of sulfur k'nown-as'lmpalpable sold by the Food Machinery Corp. was generally used. I The optimum accelerator loading was determined by -.'trtaking up one batch with the high proportion of the Santocure accelerator and a second batch with 'the'low proportion of accelerator. T hese' two base batches could then be blended to obtain intermediate levels: of accel- 'erat'onln'all cases, the batches were re-milled as usual in order to obtain the best possible dispersion.

Ourmixing procedure wasto mill therubberlin order to break it down and then add the zincoxidean'd Santocure master batches. Following this, the filler andstearic acid master batc hes which had been pre-tnii ed'were added and ,finally the sulf ur master batch.

lowed the following routineia j in general, we fol 16 carried out'on a smaller or "toy mill having 3-inch by 6-inch rolls. This was'found not to introduce any error in the procedure since the sa e results were obtained if re-milling was carried out on the standard mill.-

Forvulcanization, the. bate was placed in a mold consisting otia steel plate with eight standard rectangular. insets having sections 2.75 in hes by.5.5 inches by 0.01 inch. .The batch was, cured at the temperature of the steam in; the upper platen o an hydraulic press, 287

' 1,100 p.s.i. by compression ith alo-inch diameter by- F., for the time required, usi g a hydraulic pressure of draulic ram. The'samples. ere quenched by sion in water at room temp rature.

Dumbbell test specimens ere die, cut from the Volcanizedslabs with'a diesimil r to ASTM Die C, except that itwwas 0.75 inch at the wide part and 0.5 inch at the neck down section. These specimens give the same results" as the ordinary die specimens, except that the liability of erroris somewhat'greate'r in modulus and tear. 1 i

For tensile and modulus determinations, five modified.

dumbbell specimens were pulledon a Scott tensile testing machineat a speed of 20 inches per minute. The load at varying elongations, i.e. the v.rnodulus, is noted manually until the specimen breaks. The variousmoduli load at break and elongation of break are then calculated. -This procedure follows ASTM Standards, 1949, Part 6,

' p. 887, Specification No. D4l2-49T.-

The 'roll temperature was 150460 F. atthe start I with anincreasing variation to between l50--l72 for the entire period using cooling water through the-rolls to balance generated heat.

' After resting ove'rnightthe batch was remilled. The

- compoundwas' on the rolls at a temperature of 160" F.

and then rolled with the rolls opened sufficiently to form a small rolling bankior two minutes with continuouscross cutting and then Sheeted on at 0.090 with the rolls open to form .a sheet of sufficient thickness to nearly fill -the ASTM tensile mold cavity.

' The milling loss of the batch 3% of the added pigment.

' either absorbed from the atmosphere or water or hydration contained in the pigment lost during-the heating was found to be less than This appears to be water which occurs during milling. The millis equipped with a suction fan to avoid scattering dusts.

j The milled material was aged for 16to 24 home before for516 to 24 hours before testing;

- The-standard mill usedh'ad two 6 by 12-inch rolls roll speeds (#24 and 33.5 r.p.m. Re-milling 'was 'ofa Lupke rebound resilience tester.

. Shore hardnessis determined in duplicate on four plies .of the ends otthe tensile strips by means of 'the Shore Durometer actuated by a grossloadof two pounds; The

hardness is read after. the needlehas been in contact with the sample 10 seconds. (ASTM D676-49T, ASTM Standards, l949,'Part 6, p. l039.) Re bound resilience is determined in duplicate on four plies of the enlarged ends of the tensile strips by means (Compare -ASTM D'l'054-49T, ASTM Standards, 1949, Part 6, p. 1205 The 'Winkleman tear test specimens are died out with ASTM die type A. Four stresssstrain' and four tear-resistant specimens are made at room temperature and humidity conditions using the Scott tensile tester with the jaw separating at the rate of 20 inches per minute. (Compare ASTM D624-48, ASTM Standards, 1949, Part 6, p.

In the test for abrasion resistance in natural rubber the basic formulation was used; (Compare ASTM D394- 47, ASTM Standards, 1949, Part 6, p. 874.). One part of an anti-oxidant (Thermofiex A) defined on p. 436 of the. -Handbook of Material Trade Names, Industrial Research Services, NH. 1946, Was added in the cases indicated. The amount of Santocure accelerating agent was chosen to give an optimum cure'corresponding to the.

channel black control, that is, 25 minutes at 287 F.

The batches were mixed on a standard 6" by 12 "..mil1 and the stress-strain resistance data were obtained from standard ASTM sheets. Irtthe case of the coated matevulcanizi'ng and the vulcanized material was also'aged I rial of Example 14, the batch was mixed on the toy mill and the stress-strain data were obtained from somewhat thicker and shorter stress-strain specimens. The abrasion resistance data is the average of two determinations each on the optimum cure and a cure 5 minutes above as well as one 5 minutes below the optimum cure. The. hardness is reported as the average for the three cures of each of the abrasion discs tested. The abrasion discs were approximately 0.4 inch thick, while the tensile.

strips were from,0.08 to 0.1 inch thick. The abrasion discs being thicker were cured for. 10 minutes longer than the tensile strips. A laboratory abrasion wheel was usedto determine the relative abrasion loss.

In tests with synthetic rubber, the GRS rubber or preme 1000 is now known as SBR 1000, a hot, fatty-acid type, whereas GRSX485, the cold, rosin-acid type, later known as GRS. 100, is now known as SBR 1500.. In.v

' NH :C solutions" followed by a statement 17 preparing the GRS mixes, the mill rolls were at 95100 F. without cooling water. The rubber, zinc oxide and 1 Santocure were broken down until smooth, maintaining I a small rolling bank. Then the pigment was added and -'the ,roll opened to continue the bank. Following that, the sulfur master batch was added and the mix was cross cut and cross rolled 12 times to disperse the pigment thoroughly. 7 I

In Example 26, the mix was prepared on a Banbury mill size B, 1000 grams polymer capacity. Ordinarily, the pigment couldbe added in two increments, but that treat- I ed-with glycerine was so fluffy that fourincrements were necessary. These light powders-had a loss of about 1.5% p of the batch weight. Y Y

The mixing procedure with Example 26 was as follows.

' Roll opening 1 Time in Operation in inches minutes Crack rubber p .035 1 Two passes ,008 l. Band 075 0 Add resin .075 2 Six cross cuts 075' 2 Five passes... .005. 3 Break down rubb 085 5 Add pignient 085-. 110 11-33 i i- 8 HOUR TIME LAPSE Add other ingredients 110 3. 5 Twelve cross cuts 110 Six cross rolls.- .110 m Weight, take roll and batch temperature and sheet of! 060%065 There was a break of five to eight hours between the I mixing in the Banbury'and the mill addition of the accelerator powders and other ingredients which was done on the standard mill after removal of the batch from the Bar-ibury mixer. I

THE EXAMPLES While the above general description of the new process is believed to be. sufiiciently detailed to teach those skilled in the art how the new process can-be applied in pracdicated. In the'following examples, the terms (a) 28% NH, solution or (b) 28% ammonia solution are intended to'mean a 28% NH ammonium'hydroxide solution.- trated H01 and H 80 refer to the standard commercial.

' products having a content of 36% HCl and 93.2%, H 80 respectively. I An ammonium carbonate solution varies in the proportionof NH to CO We therefore usually refer to l of the actual composition. l

The, terms 10%, and standard silicate solution refer to aqueous solutions of N sodium silicate (e.g., a'10% standard silicate solution being comprised of 10% by weight of N sodium silicate and I 90% by weight of water).

All of these diluted silicate solutions were aged at least one day to insure uniformity of micelle size. Un-

'- aged diluted silicate solutions require different proportions of coacervating agent to give an optimum size.

The product is mall cases, substantially SiO except where mixtures are stated to form. The silica ishydrated unless anhydrous material is specified.

Concen- 18 Example 1 In this example carried out at room temperature, a mixing procedure similar to that shown in FIG. 2 was used in which the jet nozzles had a bore diameter of 0.051 centimeter. ml. of a 25% standard silicate solution was conducted through tube 1 which was two inches long and was mixed in the Y with a clustering agent comprising a 28% ammonia solution also flowing throughanother two inch leg, 2. The bottom of the Y 6 was also two inches long. This first jet was metat a point two inches below the end of the Y by a second jet from a 4 inch long tube having the same bore diameter. This second jet was an NH :C0 solution containing 0.0475 gram of NH per ml. and 0.972 gram of CO per ml. The flow rates were such that 100 ml. per minute of the 25% standard silicate solution was mixed with 25.6 ml. per minute of the 28% ammonia solution and this mixture was fixed with 37 ml. per minute of the ammonium carbonate solution. Thus'the final mixture in receptacle 5 contained 4.6% SiO 4.9% NH, and 2.1% CO, with 106 parts of ammonia to 100 parts of SiO, and 148 parts of CO to 100 parts of Na O. Theproduct from the mixing of the jets dropped into a. beaker where it was agitated continuously and when analyzed had an average particle size of 39 m In general, this mixing after the solution has set causes an increase in particle size. The product was washed thoroughly on a Buchner. funnel and dried for 18 hous at C. The ignited loss was then about 4% and there was less than 0.1% gel as in the case of another example prepared in the same manner except that the mixing system was similar to that of FIG. 6 and used two jets. The product was dried at 105 C. for 24 hours and had an ignited loss of only 4% and a pH of 8 with a particle size of 29 mp. Thejcts in this latter case had a pore diameter of 0.055 centimeters while no additional mixing was undertaken after the formation of the precipitate.

Silica products obtained by using the same three above mentioned solutions, but using the prior art slow addition technique of first pouring the clustering agent into a mixing vessel containing the silicate solution and then adding the insolubilizing agent slowly, are definitely inferior to the product obtained by the jet mixing technique of this example. We have found that the products obtained by hours at 105 C. whereas the present products retain only about 4% of water under the same drying conditions. The prior products after thorough washing had a pH of from about 9 to 10 whereas the present products gave a pH of about 8 with much less washing; The present products alsoproduce much better results when used as reinforcing agents in rubber compounding Example 2 In this example the jet mixing method was compared with our rapid-pouring method, the two products being compared as to their reinforcing actions upon rubber. In the hand-pouring method an aged 25% standard silicate solution was again used. A 28% N11 solution was mixed with the silicate solution in a proportion such that there were 196 parts of NH to each 100 parts of SiO,,. This was mixed thoroughly, giving a slight opalescence. Immediately-thereafter a 20 'B solution of-HCl was poured in rapidly in an amount equivalent to 72 parts of HCl per 100 parts of Si0 (232 parts HCl per 100 parts *ammoniawas boiled out of the product layer.

then fitlered on a 'press and washed with water until free .ofchlorides and then dried at 84 C. for 88 hours in a tray drier. electron-micrograph indicated that the product was quite-agglomerated and that the particle size was: about Na O) and pouring was completed in about '2 seconds. Mixing was continued :until the mixture thickened and then the solids were allowed to settle. I

The final solution contained 4.4% SiO -8 of NH "and 3.1% of HCl. The temperature Was-25,.C."and

vigorous mixing was maintained throughout.

.After settling, the supernatant liquid was decanted and 55 Inn. The product also contained about, 1% of gel Y so that'the P: and G particle sizeindicated was-somewhat larger, being about 82; m n-.- Ignited. loss was 9.4% and and linseedoil adsorption was 2.1 grams" gram;

-Titrata ble Na,0 was 0.52% and the HF residue. was about, 1.5%. The hardn'essby the-zinc abrasion-test was .0.o1s gm. Zn/g'm.SiO;/ minut e In a similar-test in which the proportion of the sodium I silicate ,to' insolubilizing agent was maintained constant and the ammonia was about-75% of that. required to I produce opalescence,'the reactants were mixed within one secondand the violent agitation was completfedwithin less than five seconds. The product'was dried tit-150 C.

in .a tray for threedays and had'an ignited loss of 5.8,

a bulk density fof 15.3 and an analysis of' -electrony micrographs showed that most of the particles were he tween 21 and .32 m in diameter.

In the j'et mixing. part or this test a technique was em ployed similar to thatillustrated in FIG. 1 of th'edraw-- ings. 25% standard silicate-solution was used at a flow rate of 100 ml. per minuteandmixed with a 28% NH solution 'at'a rate supplying 61- parts 0f1NHg'P6I" 100 parts of Si0 The jet of-insolubilizing agent-was a solunon containing about CO, and 5.4%NH'5 (0.0474

gram of NH per ,ml. and 0.0925 "gram. of- CO, petml.)

" The three jets each had ajbore, of 0.055 centimeter. The

mixed stream from the jets was allowed to if'alli'nto a pail without further disturbance; I The final mixture had a composition .of- 3.6% S102, 4.1% NH, and 3.6% CO The reaction was carried out at 25 C. The ratio ofNHgto SiO, was 115 parts of a I NH perf100parts of SiO; and 322 parts 01300, per 100 parts of NagO.

It was of ethylene glycol overcame this difliculty to a large extent.

' Exnn ple 3 A'system similar to Fifi; 2 was used with approximately the same composition and the product was dried at 185 C. for 24 hours. It had .an ignitedloss of 5.1 and a free water content of; from 2 to 3%. Other portions of this sample were dried at 50 C. for 96 hours 10 fwith an ignited loss of 8.1%, at'2915 -C for 24 hours with an ignited loss of 4%, and another portion-dried at 500? C. for 17 honrshad a lossof 2.4%; The portion dried at 295 C. had the best rubber reinforcing prop- I erties. The product of this test had a particle size oii the pH was 8.9. The iree water contentwasabout 1.7%

about24 and a pH of about 8. I I

' Example 4 A mixt ure was made containing 300 parts by weight of natural latex (33% solids) and 1980 parts by weight ot-a 25% standard silicate solution. To this-mixture was added .972'parts of a 28% NH "s0lution containing 1 partby weight of piperidinium-N-pcntamethylenedithiocarbamate, as accelerating agent, and 1 part by weight of phenylbetanaphthylamine, as anti-oxidant and in which 1 part of stearic acid had been emulsified. This produced incipient clustering. To this was added by jet mixing a mixture containing 17 parts of zinc "chloride and 340 parts by weightof 36% HCl solution as insolubilizing agent. The silicate mixture and the'insolubilizing agent were mixed very rapidly with jets in an arrangement similar'to that shown in FIGURE 4 of-the drawings. The

1 crumb thus producedwas filtered off and dried, If deto that shown'in FIGURE" 7, using Y tubes with diameters The product was filtered and washed free of CO, using distilled water. It was thendried at rio"; ('3. for 18 I hours. The layer was about one-quarter of aninchthick. The silica product had a particle size of 28 millir'nicrons vwith an ignited loss of'5.3.%,'a pH of .7.9 and less than 0.1% geL.

sired, fully compounded rubber obtained in this 'way'can be milled'beforevulcanization depending upon the use to which it is put. I I

. Similar procedures can he used in compounding rub- I I bet suitable'for use, as tire treads and carcasses, rubber heels, etc., as will be understood bythose skilled in the I art. This examplealso demonstrates the possibility ,of. coprecipitating our fine silica withother solids. 'In most cases; the silica is so fine that it coats the other-solids. In rubber latex, the diameter. of the rubber globules is probably 30. or. more times that of the silica particles.

Example 5 Q In this example a mixing system was employed similar of 15 mm. Tube 17 was 5 ,feet of Tygon tubing with a diameter of 0.5 inch and tube 18 was 20 feet of T'ygon tubing one inch in diameter. A 10% standard silicate 'Each of the twoproducts were incorporatedin'thei ,standardv rubber formulansing 30 parts oflith'arge as tlie acceleratorinplace of'Santocure. f I

'Thesfololowing results were obtained:

Both of these products are substantially better than any product which could be produced by a'mi'xing technique involving the slow prior art addition of insolubiliz- I ing agentto the mixture of silicate and clustering agent;

The jet mixed product was found to-absorb accelerators of the'Santocure type but t e addi ionqt a small amount solution was passed through :at a rateof 11,500 ml. per minute. 9330 m1. of ammonium carbonate solution containing 0.2070 gram of NH; per ml. and 0.0395 gram of CC, per ml. were mixed with516 'ml. of 28% ammonia and this combination was mixed with the silicate solution through the other leg of the Y. Y The reaction. was carried out at 38 C. and resulted in a final mixture of 1.5%

Si O 9.1% NI- I and 1.63% CO There were 610 parts 1 of ammonia to parts of 'silica and 350 parts of CO to 100 parts of- Na 0. The precipitate was washed on a Buchner filter using distilled water. and then dried overnight in a tray at C. in a York-dried The final .producthad a] particle size of 17 mu and contained 0.02% of gel.

Example 6 A series of tests were run' at 25 C.-in which the final solution varied in ratio by weight of NH toCO of from 2.1. to 0.5. In one, the SiO,, concentration was 2.4%, NH was 7.3% and CO was 2.3%,.the ratios were 300 parts of CO to 100 parts of N330 and 300 parts of NH5 to 100 parts of SiO,;. In this test in which 87 ml. of 28% NH; solution and 76 ml. 11 B. NH :CO (0.0698 g.

NI-I and 0.1105 g. CO /ml.) solution were first-mixed and then pouredby hand into a container simultaneously with 200 mi. of a 15% standardsilicate solution being -mixed with. a 4-bladed paddle at 600 r.p.m., a particle tion, the particle size was 311 mu. When only 250 parts of NH per 100 parts of 'SiO were used in this technique the particle size'was 19' mu. A wholly similar series of tests with 400 parts of CO to 100 parts of Na O and 200 parts NH; to 100 parts of SiO resulted in almost identi- 3 cal particle sizes of 16-17 mu but when the testwas made at 9 C. the particle size was doubled. Less NH would have produced the proper size range.

These products also were filtered on a Buchner filter,

' washed with nine volumes of distilled water and dried in the oven at about, 110 C., pulverized in a hand mill and had a gel content of about 0.010.15%.

' 7 Example 7 The apparatus used for preparing this product was similar to the arrangement shown in drawing-#7. 6750 ml. of NHgICOg solution (0.146 gram NH :0.078 gram CO per ml.) was mixed with 116 ml. of 28% ammonia solutiori in a T and then wasled-jnto reaction with 11500 ml. of 10% standard sodium silicate solution in a glass Y. The silicate inlet to the Y was mm. I.D., while'the carbonate inlet was 10 mm. ID. The outlet was 15 mm. I.D., the temperature was 32 C. This gave afinal mixture containing 1.7% SiO 5.2% NH and 2.7% CO with 300'parts of NH; per 100 parts of SiO and 500 parts of CO per 100 parts of Na O. The Reynolds num ber of the mixing procedure was 15000 with 112 feet of one inch tube leading from the final Y. The product'was washed on a vacuum box washer with distilled water, re slurr'ied, boiled, refiltered, dried at 110 C. in a tray-drier in lumps not greater than one inch in diameter. It was 4 then ball-milled by equipment constructed of 1.25 inch pipe sis-described on page: 1145 of Perrys Chemical Engi neer's Handbook, 3rd Edition, McGraw-Hill' Co., N.Y.

The product had a particle size of 16 mujwith 11.3%

ignited loss, a pH of 7.9, 0.71% Na O and 1.1% HF residue. 'The linseed oil adsorption was 1.3 grams per gram of Si().;,' and there was 0.01% gel.

One portion of thisproduct was coated with vinyltrichlorosilane by passing air through the silane at 10 to 15 I I liters per minute. The silane was heated toa vapor pressure of 300 mm. The air, containing the silane was then passed concurrently with the silica through a glass tube until the silica was completely coated as shown by the development of water repellency. After this treatment the following propertieswereestablished:

0 Particle size 'mu 19 Ignited loss 'percent 8.8 Coating ..do 15.2

When tested in the standard natural rubber formulation with optimum Saritocure and 6% stearic acid, these data were obtained the'cake of a little under 100 C. initially and up to 150 Uncoated Coated silica silica Percent Suntocure. '3. 1.0 -Modulus 300%, p.s.l. 970 1, 600 Tensile strength, rm. 3, 999 4, 100 Tear resistance, p.i 705 850 Hardness 57 62 Percent rebound 52 56 Abrasion, cc./H.P./hr 250 118 22 C. finally. This product was quite similar to that obtained in the first part of this example, but when coated with vinyl trichlorosilane and tested in rubber it had the following properties: r

The uncoated formulation required 3 Santocure while the coated combination required only 1% Santocure.

These values are within the ranges produced by rubbers reinforced by the high-grade carbon blacks used in the rubber industry.

Another similar test was run at a flow rate of 40,000 ml. developing 21' Reynolds number of 100,000. Fifty feet of half-inch plastic tubing was connected to the exit of the Y and the product was boiled at about atmospheric pressure for an hour to remove ammonia before washing. By varying the-ratio of NH to Si0 in the final mixture,

particle sizes below 20 mu could be obtained over the -range of about 251 to 290 parts of NH per parts of SiO One such product has a particle size of 18 mu, a'n ignited loss of 10.8%, an area of 134 m. /gr., a bulk density of 5.1 pounds per cubic foot and a'linseed oil adsorption of 1.45 grams per gram, a pH of 7.7, 0.3% titratable Na O and 1.3% HF residue. There was no gel residue. I

The sample was compounded in the standard formula with 2.5% Santocure, and 1% Thermofiex A as antioxidant. Upon testing the following results were obtained:

Optimum cure, minutes 25 300% elongation, p.s.i 840 Tensile strength, p.s.i 4050 Tear resistance, p.i 675 Hardness 48 Percent rebound 47 Abrasion loss, cc./H.P./hr. .231

Another similar example was carried out with the ar Particle size mu' Bulk density lbs./cu. ft 17.1 HF residue percent 1.05 pH 7.7 Percent Na O (titratable) 0.22 Wet sieve residue percent 0.03 Ignited loss do 6.53

The .dried product was micronized as described in EX- ample 7. The material which had a bulk density of 17.1 prior to reductionizing had a bulk density of 5.4 pounds per cubic foot afterwards. The particle size was unchanged and the wet sieve residue changed from 0.03% to 0.00%. Thermal conductivity was determined later on other samples.

Bulk

Igu. loss Dried sample 4. 1 Undried sample. 10. 4

v Elongation at break, percent- .Tear resistance, p .-i

A part of the undricd' slurry was spray driedin a 14' diameter conventional single fluid 'atomi'zing spray drier soldby Swenson Evaporator Company, Harvey," Illinois.

This product resembled micronized silica in that it was fflaky and slightly, translucen t. It appeared tobe very uniform and settled-slowly inwater. 'I'heptoper'ties were The tray dried material was evaluated in a standard 3 natural rubber formulation, At 3.25 parts 'ofssantocure' the compositionvulcanized satisfactorily-in minutes.

Thematerial which'was spray-dried was tested in the same'f orm'ulation at 3.25% 'Santocure with the=following results after curing 20 minutes:

' Load at'500% elongation, p.s.i. a 3120 Lead at break, p.s.i ...'-.3800

Exaniple A sample of finely divided silica produced as described in Example 7 was coated withzinc by reslurrying the wet filter cake at a concentration of 1000 parts of silica in Uneoated Zn Uneouted Zn stearatc I coated coated Tensilestrengtlr, si 3,500. 3,500 2 3,020 3,490 Modulus 300%, 13.51.. 1,500 1,320 1,000 1,110. Tear, p.i 715 600 v 010 695 24 mulation using 3.5% and 2.5% of Santocure respectively "as accelerator:

Anothersimilar testfound-that our fine silica product .could be coated with aluminum using salts such as the. chloride and nitrate, with magnesium usingsoluble mag-v nesium salt, with tin using tin chloride, with lead and with all other metals forming insoluble silicates. A tin coating was found to increase. the tensile strength, modulus and B. technical ammonium carbonatesolution containing tear resistance of'vulcanized rubber compounded therewith. I

v Example 9 In an arrangementsimilar:to'FIGURE 1, three jets were .used with flow rates based on 100 'ml. per minute of 25% standard silicate solution (11 B.) 'or 7.7 grams of SiO'; per minute. The I jetof silicate converged with that of a 28% NH, solution before'meeting a jet of 9.9"

. 0.0475 gram of NH and 0.097 gram of CO, per ml.

' Load at 3.00% elongation, p.s .i '1 620 I The silicate and carbonate jets hadbores of.0.102 centimeter and the diameter of the bore of the ammonia'jet was 0.069 centimeter. The final solution contained 2.3% of SiO 2.4% of NH, and 2.8% of CO, giving a ratio o.104 parts of NH to 100 parts of SiO: and 384'parts-- of CO: to 100 parts QfNa O. The reaction was carried out at 25 C. and the standard sodium silicate solution had beenv aged for several days. After the mixed solution set up from the jets 'it was not further agitated. It was filtered. on a Buchner-type filter and .washed'with distilled water until free of CO, as shown by the fact that the filtrate did notefferve'sce when made acid. The product was then dried at'llO". for 20 hours.

The relative fiow rates per minute were 100 ml. of the 25% standard sodium silicate solution,- 14 ml. of the 28% NH solution and 95 ml. of the NH :CO mixture.

30,000 parts of a solution of zinc acetate and zinc'chlo'ride sufficient top'rovidey5-% of Zn basedon. theedry'silica content, i.e., 147 parts of 'zinc acetate, 33 parts of Zn metal dissolved in HClJ. The product was filtered, dried 20 hours at 110 C. in an oven and milled. It was found that the sample had a zinc content of 5% by weight, and it had a particle size of 17 mu, a 'pH of 6.3, bulk density of 17.7 lbs-./cu. ft., an ignited loss 'of 9.08%,' titratable Na O of 0.27%, HF residue 2.02% and wet sieve residue of 0.01% i -In another test the filter cake of silica from-the low modulus sample described .in Example 7 was dried and reductionized and this,'as a base material,- wasslurried with a benzene solution of zinc s tearate using enough ,zinc stearate to provide 20. parts per 100 parts-0f silica. After thorough mixing and standing overnight the slurry was filtered and then-was dried 20 hours at 110 C. in

an oven and thoroughly milled without washing. This product had a particle size of 22 mu, a bulk density of 16.5 vlbs./ cu. ft., an ignited loss of 20.9%, titratable Na O 1 'of 1.64% and an HF residue of 4.9%. Since the sample wasnot wet by water, it was not possible tod'etermine either the pH or the wet sieve residue buta SoXhlet extractionindicated that a coating of 2.5 was obtained.

In the case ofthe sample coated w'ith"5% of Zn, absorption of Santocure was reduced from 0.0l3 grs./gr. of .uncoated powder to 0.009 grs./ gr. of coated powder. Dispersion in rubber was much improved by the coating.

Both samples were tested in the standard rubber for- The particle size was 19 mu.

The tests in the following four specific examplesiwere conducted using a hand-pouring technique wherein the silicate solution was introduced into a beaker and this solution was stirred while the'clustering and insolubilizing agents were added. In all cases the insolubilizing' agent was added immediately after the clustering agent and thoroughly mixed within a period ofless'than 5 seconds.- In the third example the clustering agent and insolubilizing agent were mixed before this mixture was added to the silicate solution.-

' Example 10 This test was made using 100 grams ml.) 'of a solution containing 7.74% .Na oand 7.53% Si0 (i.e.

1.0 Na OzLt) SiO Here the equilibriumopalescence ratio was 0.73 (62 ml. of 28% NH; solution to 85' ml. of'themetasilicate solution). 16 ml. of NH OH (i.e. 98% of the opalescence ratio) was addedto this solution of incipient clusters followed by 40.4 ml: concentrated HCl (0.0414 gram of HCl per ml.). In this example 216 parts. of HCl per 100 parts Na O and 203 parts of NH, per 100 parts otSiO were used. This is 1.83 acid equivalents compared to one equivalent of Na o. The original solutions were at 25 C. and the addition ofthe ammonia reduced the temperature of the mixture to,20 C. The temperature increased to 55 C. on-the addition ofthe HCl.

Thefinal solution contained 3.7% SiO 7.5% Nl-I and 8.2% of HCl. The product .was washed well with distilled water and dried in the oven ,at 100 C. It had a particlesizeof 37 m'uand 0.01% wet sieve residue.

of the 28% NH solution.

Example I] The' equilibrium opalescence ratio was found to be 0.71 for a 25% standard silicate solution and a 28% NH solution. In this test at 25 C., 100 ml. '(108 grams) of 25% standard silicate solution was mixed with 106.5 ml.

Thus, the percent of. opalescence ratio employed was 150. The solution of incipient clusters was insolubilized by the addition of 9 ml. of concentrated H 80 diluted 1:1 with' water; This example had 3.3% 'of $05 12.3% of N11 and 3.5% of H 80 in the final solution. Thus, 370 parts of NH per 100 parts of SiO and 340 parts of H 80 per 100 parts of Na O were used. This was equivalent to 2.03 acid equivalentsto one equivalent of Na' O. The precipitate was washed well with distilled water on the Buchner filter and dried. in the-oven at 100 C. The

.product had a particle size of 43 mu and no'wet sieve residue.

Example 12 RU silicate (Na O:2.4 slo was diluted to 16.6%

SiO; and 128 ml. was mixed with 567 ml. of a solution of ammonia a-nd CO (13 NH';,, 9.5% CO equivalent to 545 parts of CO per.l parts Na i) (7.35 acid equivalents) and 310 parts of NH per 100 parts SiO The equilibrium opalescence ratio was found to be 0.52. The final solution had- 3.3% S10 10.4% of Nll 'and 7.6% of C0 The test was carried out at 40 C. initial temperature and the product had a size of 41 mil with no wet sieve residue after thorough washing with distilled water and drying in an oven at 100 C. Since 16.6 gr.

NH as NH OH caused the appearance of opalescence in 128ml. of this diluted RU, the 80 grs. present in the NH :CO solution represents.485%' of the opalescence ratio. This is possible because of the greater dilution when lflfi andCo are added'as one solution.

Example- 13 100 ml. of s silicate (Na O:3,9 SiO diluted to 7.5% S102 was mixed with 50.5 ml. of 28% NH solution. This mixture is 104%, of the opalescence ratio which is 0.49. Then 16 ml. of concentratedHCl (2.96

times the Na O equivalent) was added as the insolubilizing agent. The final solution contained 4.3% of S102, 7.2%

of NH and 3.8% of HCl with 168 parts of NH per 100 parts of SiO and 345 parts of CO per 100 parts of Na O. The temperature changed from the original 25 I C. to 20C. on the addition of ammonia and'rose again to 38 C. by adding the acid. .The product had a particle size of 27 mu and no wet sieve residue after thorough washing and drying in an oven at 100 C.

Example 14 Two' processes involving a double precipitation were tested. In the first, 600 ml. of 28% ammonia solution wereadded rapidly with vigorous stirring to 300 ml. of

a 30% standard sodium silicate solution The resultant slurry was filtered and insolnbilized with 120 ml. of a off and-fixed on the filter. The acid which washed through the filter served to fix the remaining silica present in the wash water as a solution of incipient clustersof sodium silicate. This product had a smaller particle size and a 26 much reduced wet sieve residue as shown as in the following table:

A second process of partial precipitation was carried out. A solution of incipient clusters of sodium silicate was partially insolubilized in steps by the addition of acid and each precipitated portion of insoluble silica wa filtered off and washed and tested. I

In this example the temperature remained at 25 C. 400ml. of a 25% standard silicate solution was agitated in a liter beaker and 240 ml. of a 28% ammonia solution was added rapidly. Immediately afterwards varying amounts as shown in the table below of concentrated HCl (1.183 's.g., 36% HCl) were added. The precipitate was stirred for one minute, filtered and washed with three liters of tap water. Precipitates were then dried at 110 C. for 20 hours. The amount of NH corresponded to 193 parts by weight per 100 parts of SiO,. The average ignited loss of a dried product was 8.7%.-

The following table shows the product size, the wet I sieve residue, and thepercent recovery of 510; for each volume'of acid insolubilizing agent.

Total larts HCl Product Product mls. cone. by weight/ Product wet sieve recovery Pcreent* H01 100 pts. size, mu residue, gms. recovery N820 percent Based on the theoretical maximum yield of 31 grains.

percentage of gel formed drops off rather abruptly to- -a value of about 0.05% but it is not until about 15% is added that no furthergel is formed. It is evident there 'fore that any method wherein an insolubilizing agentvis added slowly to a solution of incipient clusters of sodium silicate will produce an end product which is contaminated with silica gel. It is also shown that this contamination with gel can be avoided if from about 10 to 15% of the total acid required to precipitate the silica is first added and the resulting precipitate is removed from the solution before the addition of the remaining acid.

In further tests, 240 mls. of 28% ammonia solution (equivalent to 193 parts of NH parts SiO were added to 400 mls. of 10% standard silicate solution at 25 C. undergoing agitation. Within a few seconds, varying portions of concentrated HCl were added. The precipitate was filtered and to the filtrate was added more l concentrated HCl to make a total of 80 mls;

cipitate was well washed, dried and tested. The results;

Mls. ms. 1 Prod Product Product cone. HCl/IOO, uet wetsleve- --percent HCl pts. size, residue,v recovery N5 mu pereent- 1 From table above so 3'54 I 34 0.00' 94.9 Orlginalpreclpltate.-LQ 1s. 1'1 56 11. 4-' v 68.7 Filtrulie preciplllutmun 64 283 41 0.00 I 17.0

' 1 4 p I 86.6 Original preelplthtei--. as 50 12.1" 24.0 First filtrate precipttote 35 43 0.8 44. 6 Second filtrate preeipltum".-. s4 283 42' 0.00 14.1 83.3

This procedure, of adding insolubilizing agent in two steps with an intermediate filtrating step, is not'as practical as'ou'r preferred method of thoroughly mixing in 1 the insolubilizing agent within a period not substantially. exceeding about -5 seconds and before the appearance of any' substantial amount of; an irreversible precipitate, But if a gel-contaminated product can be used for, any

purpose, it is possible to. obtain' on'e in this manner in addition to a gel-free product. Incidentally the values for the particle size given inthe first six lines of the second Each pre-- stainless steel pipe connectedto about 3.5 feet of one and table of this example as well as in the second,'fourth, and

. fifth lines of the above table are doubtless in error due to the contamination with silica gel. These values were determined by the method of Pechukas and Gage. which method is known to give low results for products contaminated by gel. The above examples show thatparticularly good results are obtained in our process when NH and CO are used either in admixture or separately as clustering. agent and insolubilizing agent. When these materials are used the ratio of NH to CO inparts by.

weight should be within the-range of from" about 0.5 :1 to 4.321 for best results. From about 2 to 4 parts-by weight of NH to 1 part of SiO; should be used while the CO should be supplied attherat'e of about 6 to 2 parts .by weight per part of Na O. For the higher silicate CO is used as insolubilizing agent in combination NH a smaller quantity of the latter is required than in the case of'faster acting (stronger acid) insolubilizing agents. As an example, when CO is used as an insolubilizmg agent, it requires only about 110 parts of NH to- 100 parts SiO whereas this requirement is increased to about 150 parts it HCl is used as insolubilizirig agent; This may be due, to the fact thatthe CO causes insolubilization to proceed at such a slow speed that the NH has more time to produce the required incipient clustering.

Example 15 I A filtered 20% standard silicate solution containing 0.0637 .gram of SiO;, and 00199:.0001 gram of Na O/ml. at aspecific gravity of 1.07 (9.5 Be.) was stored for a minimum of 24 hours at about 27 C.

A combined clustering and insolubilizing solution of NH, and CO was prepared by absorbing CO, in 28% ammonium hydroxide and diluting to a final concentration of 0.100 gram of 'NH /ml. and 0.12 gram of CO /ml. at a specific gravity. of 1.05 (6.9 B.). During the further processing, the stripping of ammonium-carbonate solution from the precipitated silica will be de-:. scribed, This stripped solutionmay be used 'asa base But too much amone-quarter inch LD. Tygon tubing followed by two feet of two inch -].D. Tygon tubi g used as an expansion nozzle to cut down the 'velocit of flow from the tubing.

.Opalescence. occurred in the expansion nozzle just after the point of connection wit final solution contained 3.9 SiO 3.26%, Nl I and 3.92% CO 'with 83 parts of NH3 per 100 partsof SiO the. smaller tubing. The

and 321 parts of. CO; per 100 parts of-Na O. The precipitated mix was allowed totall gently onto a column of slurry, the volume of which was large enough so that anypart of the mix was retained in the column for 5 minutes before it reached "the bottom of the column from which it was pumped to the stripping column. The Reynolds number for the flow in the smaller tubing was calculated to be about 67,000. 1

The precipitated silica composition was passed through a heat exchanger to exchange with the. stripper bottoms. Thus, the precipitated-silica composition was heated to about 70 C. at which temperature it entered the top of the stripping column where almost all of thearnmonia and CO; were removedduring a period of about 10 to 20 minutes. At the bottom of the-column, concentrated sulfuric acid (96% H 50 specific gravity 1.84) was'added until the stripped slurry had 'a pH of 8. -This required approximately 16 gallons perhour of concentrated acid addition of live steam and the condensation of this steam p as b a filter aid or promoter.

ganic nitrogen material and may be substituted for by diluted the precipitate to approximately .036 grams of precipitated sio /m1. of slurry. During most of the time in the stripping column the slurry was atapproximately the boiling point. The stripper bottoms after exchange came off at a temperature of about 60 C. To these hottoms was addedapproximately 0.2% by weight of SiO of Aeromine 3037 sold by American Cyanamid Company It is an alkaline, fatty, or-

cetyl trimethyl ammonium bromide.

The treated slurry was fed to a rotary filter 'where it was washed with cold water. The discharged filter cake passed to a mixing tank where it was diluted again with cold water to about15% SiO; whereas the filter cake contained about 12% Si0 This reslurriedzfilterv cake was passed directly into a second rotary filter without further treatment and was again washed {with cold water. The filter cake from thesecond filtration contained approximately 15% solids. This cake was reslurried to about 10% silica. In each washing step approximately 1 voltime of cold water was used per volume of filter cake. The final reslurry containing.l0% precipitated SiO was passed into a commercial-type-spraydrier under conditions such that the product contained volatile components equal to approximately 6% ignited loss after drying. The

inlet temperature was 600 F. and the outlet was 300 F. The material remained in the spray drier for less than a minute. the drier and that from the bag collectors was combined and, in a general case, this product was fed into a commerical Reductionizer manufactured by the Reduction Engineering Corporation. In such a device the agglomerates were reduced by circulating thematerial rapidly around a dough-nut shaped tube using compressed cold air; at about psi. From the Reductionizer the material went'to a small cyclone separator and then to stun age.

The silica from the main collection'system of,

stantially and the dustiness decreased.

'Where the product is to be used in grease, an additive such as glycerine may be sprayed .or dripped in fine streams on the combined product from the spray drier before reductionizmg. This 'final product will contain about 1.4% 'bound water and 7.7% of glycerine or about ,4. gallons per hour of commercial glycerine at 98% concentration. 'I'he'product'may then be reductionized and densitied as before. v

The products of these processes have the followin'g properties? Without With additive additive Particle size, mu 16 Bulk density, lbs lcu f 2.8 3. 5 pH 7.6 7.6 Titratable alkali as N agO, percent 0. 0.23 Residue on HF treatment,'percent 0. 54 0. 50 Oil adsorption (linseed, g./100 169 Gre se penetration at 12% 248 244 Surface area, rn. ]gr 104 Ignited loss, percent 9. 4 15. Loss at 105 (3., percent 5. 2 7. Wet screen residue, pereent 0. 00 0.

Refractive index 1. 46

' 'These products were tested in'hot (3R6 rubber, milling on an open mill for approximately 50 minutes. The 27% by volume formula. was:

Parts by weight Silica v 58.5

' These were cured at 280 F, with an optimum curing time of 45 minutes. The uncoated material had a 300% modulus of 1220 p.s.ijand a tensile strength of 28 20 p.s.i. Tear resistance was 250 p.i. With the coated material the optimum curing timeiwas 30 minutes. The 300% modulus was 670 and the tensile strcngthwas2700. The tear resistance was 285.

The product had the following properties:

U nconted Coated silica silica Particle size, s 17 15. Bulk density, lbs/cu. it 15. 9 Ignited loss, percent 9. 3 0.3 Linseed oil adsorption, grs./gr.- 1. 53 pH 9. 2 8 Titratable Na O, percent 2. 6 1. 4 HF residue, pereent 5. 0

Both samplesjwere tested by the standard rubber formulation using standard tests:

Percent rebouu It is evident that.t he coating was not very helpful. However, in a test using an Altax-Tuad accelerator system at 10 volume loading a tensile strength of 4100 p.s.i. was obtained.

Electron-micrographs of. these two fillers show that some agglomerates in the range of 400 to 500 mu were present but that there were many fine particles in'the range 4 of about 20 mu. It is believed that the fine particles are responsible for the high tear values while the agglomerates tended to reduce the tensile strength of the vulcanized rubber.

' Example 16 In this example the mixing device was somewhat similar to that shown in FIG. 7. All the reactants were at the standard temperature of 30.55105 C. A 20% standard silicate solution containing 0.0189 grams of Na O and 0.0607 grams of Si0 perml. was forced at the rate of A finely divided silica made from a solution having 1 a similar composition but with a stream of NH zl-lcl rather than NH :CO had a particle size of about 20 mu. The solution set up much more rapidly at 25 C. than did the carbonate'mixture. Mixing was done in apparatus similar. to FIG. 7 in which the tubes hadf'a radius of 0.3 cm. The streams met in a plastic fanning-out device with ancxit 1' cm. wide and 0.1 cm. high. We spread I the solution on a belt movingat '16. ft. per minute. v} Another product was formed with ammonium carbonate but using the apparatus of FIG. 6 at 200 ml. per minute. This was a hand-poured product and had considerably higher residual impurities. 2.6% Na O and 5.0% HF residue. The product was collected and washed in a 12 foot box-type filter using nine volumes of tap water per volume of cake. The product was then dried at 110 C. and air-separated with a Federal-Pneumatic Air Separator sold by Federal Classifier Systems, Chicago. The drier was a tray-type drier and the lumps were not greater than one inch thick.

Part of the product was coated with 0.65% ethylenediamine by .including 5% based on the weight of the silica in the jet of standard sodium silicate'solution.

16,000 ml. per minute through one side of a half inch stainless steel Y and 12,000 ml. of an NH :CO solution containing 0.0844 gram of NH and 0.1000 gram of CO per ml. were forced through the other side of the Y and'then mixed in 13 feet of /8 inch I.D. reinforced Tygon tubing following by a one inch I.D. nozzle one foot long. The total pressure drop was 55'p.s.i. and the temperature of precipitation was 30.9:0.3 C. The turbulence of flow was indicated byv a Reynolds number of 68,000. Opalescence appeared just atthe endof the tubing.. The final solution'contained 3.3% of SiO 3.46% of NI- I of NH;.; and 4.1% of Co -With a ratio of 105 parts of NH to parts of SiO; and 400 parts of CO; to 100 parts of Na O. 4 p g The solution discharged onto a moving belt where it was held quiescent for 32 seconds and then discharged into a tank and thence into the suction end of an Eco were then spray-dried at standard conditions including I with water.

I an extra period of drying for one hour by passing hot air at 300' F. through the bag-house after the feed had been tcrminated- The spray-drier was a Swenson research-type spray-drier with a feed pressure of 50:5 p.s.i.,an inlet This product'was tested in natural rubber which when unloaded had an optimum .cure time of 1'50'minute's, a

vA GRS rubber composition with 3.5 parts of San to cure accelerator was undercured but still had a tensile strengthof 3060' p.s.i. and a' modulus 'of' 1060 p.s.i.

A similar solution was made up by passing the; respective solutions into separate legs of a one-half inch stainless steel Y followed by 13 ft. of inch I.D. Tygon tubing connected to one foot of one inch in Ty'gori tubing. Opalescence occurred in this expansion nozzle.

The filter cake in the press was washed with 2 volumes I of cold tap water., The cake was discharged and reslurried with a further 2 volumes of cold tapwater and acidified to a pH of 5.5 using sulfuric acid diluted 1' to 3 This was refiltered on the Shriver filter press, again reslurried and the pH raised to 8.2. This slurry was then fed to-a spray drien'with an inlet temperature of about-1100 R, an outlet temperature of 315 F. andan atomization pressureof 55 p.s.i. After drying, the product was reductioniz ed and has the properties given in' the following table:

Withontiuhlit'ive Withudditlvo Particle size, mu 10-20 11-18. Bulk density, ibSl/Cll. it 10-12 10-12. g gli'. 4.510 8-10.

l0, content, percent (anhydrous Greater than Less than 91.3.

basis). 05. Free water, percent 2-10 20. BOlrlllld (water, moles oi Slo /moles 4-8 tlggater than Surface area, un /gr. 0.5-1.5.

NinO content, .percen Tensile strength in natural Greater than Greater than rubber, p.s.i. 3700.- ensiie strength in GR-S rubber, Grenterthun Greater than p.s.i. 2500. 2500; I Modulus in natural rubber, p.s.i Grgater than 1 ASTM linseed oil absorption in 1 Greater than Greater than lbs/1001M. 180. 100. 325 M wet sieve residue, percent, 0.00.. 0.00. Grease/:giekening power, Less-than 245"; Less than 245.

mm. i v HF residue I Less than 4%.. Less than 4%. Ignited loss, percent 5-15 -20. Reflective index 300% moduluspsi. "16 00 500% modulus p.s.i. u... 3160 Tensile strength p.s.i. -1 3950' Elongation at break percent I 620 .Tear'resistance p'.i. l 840- Shorehardness v '64 Rebound percent 58.5

In another example, using a similar composition, we

exemplify the commercial potential of the fine'ly divided silica formed by our invention. The following example shows the method of preparation of a high area-low pH silica. up to. about 400 ml/gram without the formation of gelparticles may be prepared At the same time the pH maybe varied from below 4.5 to above 9.0.

The 'difficult problem of separationof the silica from a highly ammoniacal mother liquor is overcome by diluting the original slurrywith cold tap water thusreducing the original concentration of the salts during initial filtration.

Thus the cake as it. was formed contained 1 much less impuritiesand localized shrinking did nottake place. As a further aid to filtration, it was found that between 1 and 2 volumes of wash water were required for the first wash on the filter. The. mixing system of FIG. 7 was employed; After washing and filtering, the filter cakerwas reslurried to a 10% slurry and stored in polyethylene lined drums until it was spray dried using an inlettemperature of 1160 F., 305 F. outlet temperature, and an air pressure of 55 :5 p.s.i. Theslurry pressure I was 58:5 p.s.i.

I The product could-be densified by passing through an-evacuation system at about 8 p.s.i. absolute. and the powder compressed at about 5 p.s.i. Where the product is to be used in grease, glycerine may be added by spraying or dripping fine streams on'the product before reductionizing. I

By' using suitablev modifications of this process as indicated in the previous examples, the products have the properties indicated below;

cubic foot.

This product had a particle size of 10 mu and an ignited loss of 11.2%. The pH was 6.3. The bulk density was 11.9 pounds per cubic foot but when reductionized it had a bulk density vof about 2 pounds per The area was 390 mF/gr. The linseed oil adsorption was 219 pounds per 100 pounds product and the grease thickening power-was 253 mm./ 10. There was no sieve residue. The HF residue was about. 0.35

and the N320 titratable content was 0.06%. Free water was about 6%.

Example 17 SD; per ml. was placed in a cylindrical plastic vessel v18 inches long by 5 inches in diameter with -3 pairs of battles alternated at angles,- each bafile being'about 1 inchhigh, 1% inches long andv A inch thick. They were set at levels approximately one inch apart. The impeller was a /2 inch stainless steel rod with 2' blades curved and located between each pair of baflles on the bottom of the'vessel. This rotated at a speed of 3450 r.'p.m. The temperature of the reactants was 32 C.

A volume of 342 ml. of .NH31CO2 solution containing 0.0938 grams of CO per ml. and 0.1202 grams of NH per ml. and 254 ml. of 28% NH;, ammonia solution were added to the vessel with a stirrer in operation in such proportion that there was 400 parts of CO per parts of N330 and 410 parts of ammonia per 100 parts of SiO;. The final concentration of SD; was 1.8%, 7.4% NH;, and 2.2% CO Afterreaction the precipitate was filtered and washed with 9 volumes of distilled wash water. It

Silicas having areas ranging from 25 m-F/gram' Particle size, mu 19 Bulk density, lbs/cu. ft. 4.3 Ignited loss percent 10.38 Wet sieve residue percent -a--'- '-a- 0.00 pH 8.0 Titratable Na O percent 0.73 HF residue percent 2.05

This material was tested in the standard formulation with Santocure at 3.5 parts by weight. composition cured in '15 minutes at 287 F. The following results were obtained:

Particle size, mu 18 Bulk density, lbs/cu. ft. 7.1 Titratable alkali as Na o percent:

(cold) i- 0.58 (hot) 0.75 Actual Na,0 percent 0.34 Ignition loss percent a. h 11.3 pH- 7.9 Surface area mf /gr. 103 Wet sieve residue percent 0.00 C-aO percent a 0.55 Mg() percent 1 0.21 A1303 percent I mo, percent 0.76

300% modulus p.s.i. ..a..- 1715 500% modulus p.s.i 3130 Load at break p.s.i. 3870 Elongation at break percent 595 Tear resistance p.i 685 Shore hardness 1 v 54 Rebound percent 63 agent was increased and less gel was formed.

No NmCO; NugCOt Parts N H Size, Wet sieve, Size; Wet sieve, mu percent mu percent 500 33 0. 00 0. 00 475 29 0.00 26 0. 00 450 19 0.00 20 0. 00 425 13 0. 01 17 0. 02 400 gel 28. 0 gel 2. 9

Example 18 Using a one-half inch diameter stainless steel Y followed by 24 feet of inch I.D. heavy-walled Tygon tubing, somewhat similar to the arrangement shown in FIG. 7, a solution of 10% standard silicatesolution and NH :CO was mixed and the slurry discharged through a one foot long piece of one inch Tygon tubing onto a By the addition of sodium carbonate the effective concentration of the clustering a hardness of 51 and a rebound of 61%.

Before boiling the particle size was 17 mu and after 34 Additional drying at about 350 F. was maintained for 3 hours. 1

This material had. the following properties:

In a similar solution using the equipment of FIG. 5 the product was prepared at an opalescence ratio of 23%. This product was boiled 1 hour before washing, instead of boiling the slurry. It was dried in a tray drier instead of being spray dried.

The product was ball-milled and reductionized and had a particle size of 20 mu with a bulk density of 4.1 p.c.f. The ignited loss was 10.5%, the pH was 7.8 and titratable Ne t)l 0.48%. HF residue was 1.02%. There .was no ge p 1 The product was tested in the standard rubber formulation using 3% of Santocure. The o timum curing time was 20 minutes and provided a tensile strength of 3700 p.S.i., a 300% modulus of 1600 p.s.i., a tear of 520 p.i.,

boiling, 18 mu. The product was White, soft and had a tendency to form pellets. The slurry did not set up on the belt itself.

In a formulation cured 25 minutes with 2.5 parts of Santocure accelerator the following properties were belt moving at 30 feet per minute. The sodium silicate containing 0.01014 grams of NagO per cc. and 0.0328 grams of SiO; per cc. was used at a rate of 25,000 parts by volume per minute. 12,900 parts by volume of NH,:c0,, solution containing 0.1260 grams of ammonia per cc. and 0.0933 grams of C0, per cc. was mixed with 1280 parts by volume of 28% NH, solution just prior to mixing with the silicate solution in the Y. The final mixture contained 2.0% of SiO,, 4.9% NH, and 3.0% C0 The mix proportions were about 250 parts of NH per 100 parts of SiO, and 500 parts of CO, per 100 parts of Na O. The temperature was about 32.2i-.5 C. with a total pressure drop of 63 p.s.i.

The slurry was boiled for one hour, filtered within 4 hours and washed with tap water andthen spray-dried in the Swenson researchyapparatus. The slurry had a Baum gravity of 7.5 when sprayed into the drier. The

inlet temperature was 680-720 F. and the outlet tem- Rebound percent -a 1 Example 19 In this example 9000 ml. per minute of a commercial, undiluted clarified E" sodium silicate containing .1203 grams of Na o and .3872 grams of 810, per ml. were caused to react with 19,700 ml. per minute of an NH CO, solution containing .0688 grams of NH; and

.1040 grams of CO, per ml. by causing the two liquids to flow into the arms of a one-half inch stainless steel Y followed by two feet of one inch LD. Tygon tubing. The mixed solution contained 10.5% SiO 4% of NH and 6.1% of CO corresponding to 39 parts of NH per parts of SiO and parts of CO per 100 parts of Na O. Precipitation of the silica occurred in the tubing and the set-tip mixture issued from the end of the tubing in the form of one inch diameter cylinders approximately 3" to 5" long. These cylinders remained on a moving belt with some syneresis for about 30 seconds and were then collected from the end of the belt in a container. The reaction occurred at 32.3 C. with a total pressure drop of 46 p.s.i.

The' lumps'of precipitate were reslurried with tap water to about 4% solids and filtered-on a vacuum-type box The dewatered cakes were then washed with an excess of 70 C. tap water and the washed cakes after dewatering were reslurried with tap water to about 8% solids and spray-dried with an air inlet temperature of 1100" R, an air outlet temperature of 295 F. and an cu're time was minutes. 1140'.p.s.i., the. tensile strength was 3650 p.s.i: -and the .tear re'sista'ncewas'67'5 p.i.,

atomization pressure of 45 p.s.i. The slurry feed-pressure The product .had the following propertieszf .When tested in natural rubber using thelstandard formulation at a .Santocure value of 3.5% the optimum The, 300% moduluswas Example I I contained 0.0637 grams of SiO, and,0.0l98 grams of Na:() per ml. were caused to react with 12,120 parts by a volume of NH :CO solution containing .0.0840,.grams of NH:, and 0.1041v grams of CO per ml. by passingthe respective solutions into the separate legs of a one-half inch stainless steel Y followed by 13 feet. of inch I'.D.

Tygon tubing connected to one foot of one inch I.D.

Tygon tubing. Opalescence occurred in this expansion I nozzle. The final solution contained 3.4% OfSiO- 3.3%

of NH and 4.1% of CO, thus having 100.parts of NH per 100 parts of SiO and 400 parts of CO', per 100 parts of Na O. The reaction temperature was 305' and the pressure was 44 p.s.i.

\ I for at least seconds.

The precipitated product-was diluted with 2 volumes of cold tap water, thoroughly mixed and then filtered in a Shriver filter press containing 18 frames, each'24" x 24". The filter cake in the press was then washed with 2 volumes of cold tap water and the cake was discharged and reslurried with a further 2 volumes of 'cold'tap water and acidified to a pH of 5.5 using sulfuric acid diluted 1 to 3 with wated. The acidified slurry at a pH of 4.6 was refiltered through the Shriver press and again washed with 2 volumes of cold water. The filter cakefwasdis charged from the press, reslurried, diluted to 6% solids and caustic soda added to raise the pH. to 8.2. This slurry was fed into the spray drier'using an inlet temperature of about 1100 R, an outlet temperature of 315- F. and an atomization pressure of 55 p.s.i. The feed pressure was 60 p.s.i. After drying the a product was vreductionized and had the properties given in the following table.

Particle size, mu

Bulk density, lbs/cu. ft. 3.2 .pH 8.0 SiO (anhydrous basis) percent 99.5

Loss at 105 C. wt. percent 4.5

Ignited loss, wt. percent 9.6

Area mF/gr. 27.9

Titratable N2 0, wt. percent 0.25

HFresidue, wt. percent 0.73

Wetsieve residue, wt. percent 0.00 ASTM linseed an absorption, lbs/100 lbs. 202

v Grease thickening-power,'rnm./l0 strokes 241 The product could be densified by passingfthrough an evacuation system at about 8 p.s.i. absolute and the pow- Particle size, 'mu' Ignited loss percent 9.1 Bulk density, lbsf/cu. ft. 7.2 pH..'.' e 8.0 Titratable Na O percent 0. 69 HF'r'esidue percent a 1.75 Wet sieve residue percent 0.00 Surface area nf/gr. 143 Linseed oil adsorption grs./gr. 2.1 8 Grease penetration value 346 to be used in grease glyccrinemay be added by spraying or dripping fin'e' streams on the'product before reductionizing.

By using suitable modifications of this process as indicated in the previous examples, the products have the properties-indicated below. I

with additive without additive 'Partiele size, ,4 10-20 11-18. Bulk density, lbs/cu. it 10-12. git. 4 8-10. I92 acaitcnt, percent (anhydrous Less than 01.3.

)as s 1 Free water, percent 2-6. Bound water, moles of Slo /moles Greater than 12.

of Th0. 1 Filler form I Porous flocs. Surface area, m.,/gr, 00-300. N010 content, perccn 0.5-1.5.

In this example 16,000'parts by volume of a'20% standard silicate solution which was aged overnight and Tonsqe strength in natural rubber, Greater than p.s. Tensile strength in GR-S rubber,

p.s.i. Modulus in natural rubber, p.s.i..-

Greater than Greater than Greater than (Jrcutor than AS'IM linseed oil absorption in tireuter'thun I Grcutcrthun lbs/100 lbs. m0. 325 M wet sieve residue, percent"- 0.00. Grease'thickoning power, mun/10- 55 than 245... Less than 245. HF'resldua. Less than 4%.... Less than 4%.

Ignited loss, percent... '525.-.- 10-20. Refractive index 1.40

Example 21 Exemplifying the commercial potentialof the finely divided silica formed by our invention, the following example shows the method of preparation of a high-arealow pH silica. We are thus able to prepare silicas having.

. areas ranging from 25 mf gr; up to about ;400 mF/gr.

without the. formation of. gel particles as shownby our gel'screen test. At the same time the pH may be varied from below about 4.5 to above about 9.0.

The most difficult problem in the preparation of a high area silica by, our. process is the. separation of the silica from its highly ar'nmoniacal mother liquor in its original precipitated formwithout substantial loss of surface area.. This means that the original slurry of precipitated silica should not be heated. Our experience has shown that the stumbling block to'this separation and washing of the product is the inability to form cakes in the filter press, which, on washing, do not crack badly,

thus making the displacement with water ineffectual and I 3 resulting in a prohibitiveloss of ammonia.

With the proposed product,.this ammonia loss is compounded since the subsequent step is a neutralization of the fixed alkali giving a low pH. This would require" the-neutralization of the remaining ammonia on the filter. cake before the alkali would be neutralized. It would not only require I a large additional amount of acid but also intensify the 1 and 2.5 volumes of wash 'water were required for the first wash on the filter.

At 29 C. 17,000 parts by volume of 20% B sodium silicate solution, at 0.0198 g. Na' O and 0.0638 g. SiO, per ml. wasused with 12,250 parts by volume of NH :CO solution containing 0.0997 g. NH and 0.0878, g. C0, per ml. of solution.

The final composition contained 3.5% SiO 3.9% NH and 3.4% CO, with a ratio of 112 parts NH; to SiO and 320 parts CO; to 100 parts Na O. The pressure drop was '45 p.s.i. through the system and opalescence occurred in the'nozzle. The standard mixing device s milar to FIG. 7 was used with A inch I.D. stainless 

2. IN A PROCESS FOR THE MANUFACTURE OF FINELY DIVIDED SILICA PRODUCTS WHEREIN AN AQUEOUS SOLUTION OF SODIUM SILICATE HAVING A WEIGHT PERCENT RATIO OF NA2O TO SIO2 WITHIN THE RANGE OF FROM ABOUT 2:1 TO 1:4 AND A SODIUM SILICATE CONCENTRATION OF FROM ABOUT 0.5 TO 30% SIO2 IS CONTACTED WITH A COACERVATING AGENT CAPABLE OF CLUSTERING AQUEOUS SODIUM SILICATE SOLUTIONS AND AN INSOLUBILIZING AGENT CONSISTING OF AN ACIDIC MATERIAL HAVING AN ANION OF AN ACID STRONGER THAN SILICIC ACID AND CAPABLE OF PRECIPITATING SUBSTANTIALLY PURE SILICA FROM THE MIXTURE, THE IMPROVEMENT WHICH COMPRISES: (A) INTRODUCING SAID COACERVATING AGENT IN A QUANTITY WITHIN THE RANGE OF 20-500% OF THE EQUILIBRIUM OPALESCENCE RATIO, (B) INTRODUCING THE INSOLUBILIZING AGENT IN A QUANTITY SUFFICIENT TO PRICIPITATE GEL-FREE SILICA, (C) MAINTAINING THE ENVIRONMENTAL CONDITIONS SUBSTANTIALLY UNIFORM IN THE ABOVE MIXTURE WHILE THE FINELY DIVIDED SILICA PRODUCTS ARE IN FORMATION BY; (D) MIXING SAID COACERVATING AGENT WITH SAID SODIUM SILICATE SOLUTION NOT LATER THAN SAID INSOLUBILIZING AGENT IS MIXED THEREWITH, (E) COMPLETING THE MIXING IN OF THE INSOLUBILIZING AGENT BEFORE THE APPEARANCE OF ANY SUBSTANTIAL AMOUNT OF A SILICA PRECIPITATE, AND IN ANY EVENT WITHIN A PERIOD NOT SUBSTANTIALLY EXCEEDING FIVE SECONDS, (F) FOLLOWING COMPLETION OF THE MIXING IN OF THE INSOLUBILIZING AGENT, RECOVERING THE FINELY DIVIDED SILICA THEREBY PRODUCED.
 9. FINELY DIVIDED, PRECIPITATED SILICA PIGMENT CAPABLE OF PRODUCING VULCANIZED RUBBER PRODUCTS HAVING A MODULUS OF AT LEAST 1300 P.S.I. AT 300% EXTENSION AND A TENSILE STRENGTH ABOVE 3700 P.S.I. WHEN INCORPORATED IN AN AMOUNT OF 42.6 PARTS WITH 100 PARTS OF NATURAL RUBBER, 5 PARTS ZNO, 3 PARTS SULPHUR, AN OPTIMUM SANTOCURE LEVEL BETWEEN 0.75 AND 4.5 PARTS AND 3 PARTS OF STEARIC ACID AND VULCANIZED AT 287*F., SAID PIGMENT HAVING A PARTICLE SIZE WITHIN THE RANGE OF 10-20 MU, A PH WITHIN THE RANGE OF 4.5-10, AN SIO2 CONTENT GREATER THAN 95% ON AN ANHYDROUS BASIS, A 0.00% 325 MESH WET SIEVE RESIDUE, AN HF RESIDUE LESS THAN 4% AND AN IGNITED LOSS OF BETWEEN ABOUT 5 AND 15%. 